KEY POINTS
- The study clinically implemented the motorized MR-conditional OCTAVIUS 4D phantom with the 1500 MR ionization chamber array on an Elekta Unity 1.5 T MR-LINAC for three-dimensional patient-specific SBRT quality assurance.
- The evaluation included 40 clinical step-and-shoot SBRT plans: 33 prostate, 2 pancreas and 5 liver cases. Plans contained 11–15 beam angles and included irregular spacing, repeated gantry repositioning and clockwise/counterclockwise transitions.
- Three-dimensional measured dose was compared with Monaco treatment-planning calculations using a local 2.5%/2.5 mm gamma criterion with a 10% dose threshold. Mean gamma passing rate was 98.3%, and all 40 plans exceeded the institutional ≥95% acceptance threshold.
- Independent inclinometer measurements showed close agreement between gantry, phantom and software-reported angles, with maximum deviations below 0.6° and no systematic angular bias.
- Clinically relevant asymmetric targets produced radial off-axis measurement distances up to 11.9 cm, yet no visible relationship between off-axis position and gamma passing rate was observed.
- The system also maintained agreement during irregular beam-angle spacing and changes in gantry rotation direction, supporting reliable synchronization between the motorized phantom and Unity gantry during step-and-shoot delivery.
- Important limitations remain. Targets beyond approximately 12 cm off-axis, extremely small fields and highly modulated extreme delivery conditions were not characterized, and the authors describe phantom positioning with the supplied marker plate as relatively cumbersome.
CLINICAL TAKEAWAY
For centers performing SBRT on a 1.5 T Unity MR-LINAC, OCTAVIUS 4D appears technically suitable for routine rotational three-dimensional patient-specific QA. This is an implementation and performance validation study rather than evidence that one QA strategy is superior to alternatives.